LEDs (Light Emitting Diodes), with their recent improvements in efficiency, have come into widespread use in backlights of display devices and in lighting equipment, as energy saving light sources over light bulbs and fluorescent lights. In such applications, energy efficiency is very important.
LEDs, especially gallium nitride LEDs, are subject to failure due to electrostatic discharge, that is, has a property of low reverse breakdown voltage. As a countermeasure, a technique of disposing a zener diode in inverse parallel with an LED is disclosed (see, for example, Patent Citation PTL 1).
In this case, the LED is protected from an overvoltage because with a forward overvoltage, an overcurrent is bypassed by a zener-breakdown, while with a reverse overvoltage, an overcurrent is bypassed as a regular forward. The LED has a forward voltage lower than a zener breakdown voltage of the zener diode. Thus, when the forward voltage is applied to the LED, no current flows through the zener diode, resulting in no energy loss. Compared with resistors, however, zener diodes cannot be fabricated easily and involve more laborious work in, for example, mounting onto the LEDs. Moreover, zener diodes are less reliable than resistors in the long run.
Furthermore, a technique of coupling resistors to respective LEDs in parallel is disclosed (see, for example, PTL 2 or PTL 3).
In a technique disclosed in PTL 2, resistors are coupled in parallel to respective LEDs that are coupled to each other in series. Each resistor serves as a bypass resistor to ensure that breakage of one of the LEDs does not turn the other LEDs unlit. For the bypass resistors to serve their purpose, a current large enough to light the unbroken other LEDs needs to flow through the bypass resistors. Thus, the resistances of the resistors to be used need to be low. This poses a problem of large energy loss in the current flowing through the bypass resistors.
In the technique disclosed in PTL 3, in an assemblage LED device in which a plurality of LED chips are disposed in a single package, variable resistors are disposed in parallel to respective LED chips so as to adjust the luminance of the respective LED chips. In this case, the resistance of each resistor needs to be low enough to allow a current to flow therethrough in an amount that affects the luminance of the LED chip. That is, a problem of large energy loss arises.
Furthermore, a technique of disposing a thick film resistor in a LED package is disclosed as an example of forming a resistor to be coupled to an LED (see, for example, PTL 4).